This invention is in the field of zeolite-type membranes whose selectivity is improved by adsorption of a modifying agent within and/or on the membrane. Useful modifying agents include ammonia.
Zeolites are largely composed of Si, Al and O and have a three-dimensional microporous crystal framework structure largely of [SiO4]4− and [AlO4]5− tetrahedral units. To balance negative charge due to the incorporation of Al atoms in the framework, cations are incorporated into the cavities and channels of the framework. The cages, channels and cavities created by the crystal framework can permit separation of mixtures of molecules based on their effective sizes.
Different zeolites may have different Si/Al ratios and the tetrahedra can also be isostructurally substituted by other elements such as B, Fe, Ga, Ge, Mn, P, and Ti. In an extreme case, zeolite molecular sieves may have a Si/Al ratio approaching infinity. Silica molecular sieves do not have a net negative framework charge, exhibit a high degree of hydrophobicity, and have no ion exchange capacity. Silicalite-1, and silicalite-2, and Deca-dodecasil 3R (DD3R) are examples of silica molecular sieves.
Aluminophosphate (AlPO) molecular sieves are largely composed of Al, P and O and have three-dimensional microporous crystal framework structure largely of [PO4]3− and [AlO4]5− tetrahedral units. Silicoaluminophosphate (SAPO) molecular sieves are largely composed of Si, Al, P and O and have a three-dimensional microporous crystal framework structure largely of [PO4]3−, [AlO4]5− and [SiO4]4− tetrahedral units. Molecular sieve framework structures are discussed in more detail by Baerlocher et al. (Baerlocher, Ch., et al., 2001, Atlas of Framework Structures Types, 5th revised ed., Elsevier, Amsterdam).
Molecular sieve membranes have been proposed for use in separating gas mixtures. Several types of molecular sieve membranes have been tested for separation of mixtures of CO2 and CH4, including FAU zeolite membranes (Kusakabe, K. et al. 1997, Ind. Eng. Chem. Res., 36, 649; Weh, K. et al., 2002, Micropor. Mesopor. Mater. 54, 47), MFI zeolite membranes (Van der Broeke, L. J. P. et al., 1999, Chem. Eng. Sci., 54, 259; Poshusta, J. C. et al., 1999, J. Membr. Sci., 160, 115), SAPO-34 membranes (Poshusta, J. C. et al., 1998, Ind. Eng. Chem. Res., 37, 3924; Poshusta, J. C. et al., 2000, AlChE Journal., 46(4), 779), LTA zeolite membranes (Aoki K. et al., 1998, J. Membr. Sci., 141, 197), ETS-4 zeolite membranes (Guan, G. et al., 2002, Sep. Sci. Technol., 37, 1031), and DD3R membranes (Tomita, T. et al., Micropor. Mesopor. Mater., 2004, 68, 71-75). Tomita et al. reported a CO2/CH4 separation selectivity of 220 for a DD3R membrane with a 50/50 gas mixture at 301K. Poshusta et al. (2000, supra) reported a CO2/CH4 separation selectivity of 36 for a SAPO-34 membrane with a 50/50 gas mixture at 300 K. Falconer et al. (U.S. patent application Ser. No. 10/805,183) reported CO2/CH4 separation selectivities in excess of 60 for a SAPO-34 membrane with a 50/50 gas mixture at 297 K and a 138 KPa pressure drop. Poshusta et al. (1999, supra) reported a CO2/CH4 separation selectivity of 5.5 for an H-ZSM-5 (MFI structure) membrane with a 50/50 gas mixture at 301K.
Adsorption of ammonia and other compounds on molecular sieves has been reported. Zeolites have been treated with ammonia for the purposes of measuring zeolite acidity. Zeolite acidity is measured from desorption of sorbed ammonia (Dyer, A., An Introduction to Molecular Sieves, 1988, John Wiley and Sons, New York, p. 124). SAPOs have also been treated with ammonia to measure acidic site population. U.S. Pat. No. 5,248,647, to Barger et al., reports measuring the acidic site population of silicoaluminophosphates after calcination by contacting the silicoaluminophosphate with a mixture of ammonia and helium and then desorbing the ammonia.
Treatment of metallophosphate molecular sieves with ammonia has been reported to stabilize the molecular sieves. U.S. Patent Publication 2003/0149321A1 to Mees at al. and Mees et al. (Mees, F. D. P, et al. 2003, Chem. Commun., 1, pp 44-45) report stabilization of metalloaluminophosphate molecular sieves, including SAPO 34, through treatment with ammonia. Mees et al. report that the ammonia is chemisorbed to acid catalytic sites of the metalloaluminophosphate molecular sieve. Buchholz et al. report a two step adsorption process for SAPO-34 and SAPO-37 (Buchholz et al., 2004, J. Phys. Chemistry, Vol. 108, pp 3107-3113). As reported, the first step consists of an adsorption of ammonia exclusively at Bronsted acidic bridging OH groups (SiOHAl) leading to the formation of ammonium ions (NH4 form). The second ammoniation step, which was reported to occur at higher ammonia coverage, consists of a coordination of ammonia molecules to framework Al atoms.
Mees et al. (U.S. Pat. No. 6,756,516) also report stabilization of metalloaluminophosphate molecular sieves by treatment with one or more nitrogen containing compounds selected from the group consisting of amines, monocyclic heterocyclic compounds, organonitrile compounds and mixtures thereof so that the nitrogen containing compound is chemisorbed and/or physisorbed with the molecular sieve.
U.S. Pat. No. 6,051,746, to Sun et al., reports modification of small pore molecular sieve catalysts by adsorption of polynuclear aromatic heterocyclic compounds onto the catalyst. The modified catalysts were reported to have increased selectivity to olefins. The modifiers comprise polynuclear aromatic heterocyclic compounds with at least three interconnected ring structures having at least one nitrogen atom as a ring substituent, each ring structure having at least five ring members and quaternary salts thereof.
Ammonium cation exchange of zeolites is also known to the art (Dyer, A., An Introduction to Molecular Sieves, 1988, John Wiley and Sons, New York, p. 121). The ammoniated zeolites can then be calcined to produce the hydrogen form of the zeolite.
U.S. Pat. No. 6,051,745 reports nitridation of silicoaluminophosphates which can be achieved with mixtures of ammonia and hydrogen.